Re: [PATCH] blame: add the ability to ignore commits

"Randall S. Becker" [off-list ref] writes:

On January 14, 2019 12:46, Junio C Hamano wrote:

quoted

Barret Rhoden [off-list ref] writes:

quoted

On 2019-01-10 at 14:29 Junio C Hamano [off-list ref] wrote:

quoted

quoted

For instance, commit X does this:

-foo(x,y);
+foo(x,y,z);

Then commit Y comes along to reformat it:

-foo(x,y,z);
+foo(x, y, z);

And the history / rev-list for the file looks like:

---O---A---X---B---C---D---Y---E---F

I want to ignore/skip Y and see X in the blame output.

If you skip Y, the altered history would have "foo(x, y, z)" in E,
"foo(x,y,z)" in X, and "foo(x,y)" in A.  If you start blaming from F,
you'd get E as the commit that explains the latest state.  If you do
not skip Y, you'd get Y.  I am not sure how you'd get X in either
case.

...

From my own community, this came up also. The intent was to show everyone
who touched a particular line, throughout history, not just the current one.
Perhaps that is what Barret is going for.

I think I now understand what is going on.  In short, what Barret
wanted is way more ambitious than "blame in a hypothetical history
that lacks certain commits".  There is no similiarity with existing
"-S <revs>" option that I asked about in my initial review.  This is
quite a different beast (and I kind-a like it ;-).

Instead of "skipping" Y, Barret wants to pretend that the effect of
commit Y, in addition to the commit Y itself, never existed in the
history.  Even when blaming back from F (where it has "foo(x, y,
z)", which is the same as E and down to Y), the algorithm wants to
pretend as if E and F had that line in "foo(x,y,z)" form, i.e.
before Y touched it, and also pretend that the user started blaming
a line in that shape.

As there is no general solution [*1*] to "how did this line looked
like before this commit", such a blame may not have a meaningful
answer.  When Y added a block of text without removing anything
(i.e. "git show Y" has no line that begins with '-'), and when the
user asked to blame that block of text in F, there is no sensible
answer to the question: "what got replaced by Y in this text?", so a
blame that "ignores" Y would not be able to answer "where did these
lines come from?" question.  But in some cases (e.g. when "git show
Y" has only one '-' line, immediately followed by '+' line, both
with quite a similar contents and no other change in the patch---it
is highly plausible that in such a change, the former became the
latter, like the example in Barret's message I was responding to),
we can say "this was how the line that was modified in Y looked like
before Y" and blaming while ignoring Y can have a reasonable answer.


[Footnote]

*1* Here, I use the word "solution" in the same sense as "solution
to a math equation", not in the sense to describe the steps to
derive that "solution".